Pneumocystis pneumonia (PcP) remains an important cause of morbidity and mortality among immune compromised patients. Despite the availability of effective prophylaxis, each year 5,000-10,000 cases occur in the U.S. and more than 400,000 cases occur worldwide. With a mortality rate of 10-20% there are thousands of deaths and significant morbidity as a result of PCP. Furthermore, patients requiring ventilator support have a mortality rate of 50% or even higher. Therefore, there is a definite need for novel therapeutic strategies that quickly eradicate the organism in the absence of inflammation and injury. In this exploratory/developmental grant, we propose to determine whether activation of inhibitory T cell receptors limits the ability of CD8+ T cells to fight PC infection in hosts with impaired CD4 function. In the absence of CD4 cells, the CD8+ T cell population recruited to the lungs lacks effector activity and may even suppress residual lung immunity allowing, Pc to grow and disease to progress. However, it has also been demonstrated that CD8+ T cells can be activated to eradicate PC infection in the absence CD4+ T cells. We have found that the majority of CD8+ T cells recruited to the lungs during PC infection express the inhibitory receptor PD-1, which is also a marker of T cell exhaustion. PD-1 signaling inhibits T effector function and is also important for Treg function. Thus, we hypothesize that CD8 cells in the lungs of chronically PC-infected lungs adopt an exhausted and/or suppressor phenotype, which limits the host defense function of these cells. By blocking inhibitory receptor signaling we believe that these cells can be rescued and effector function restored. The overall goal of this exploratory /developmental grant is to determine whether T cell inhibitory receptors limit the anti-PC effector function of CD8+ T cells and contribute to an immunosuppressive lung environment in hosts with impaired CD4+ T cell function. To accomplish this goal we will complete the following Specific Aims: 1) To determine whether inhibitory receptor expression defines functionally distinct CD8+ T cell subsets during PcP; 2) To determine whether inhibitory receptor blockade restores CD8+ T cell effector function and anti-PC host defense. The proposed research will enhance our understanding of host defense against PC infection, and has the potential to lead to novel therapeutic strategies that can be translated to improve patient care.